1. Field of the Invention
The present invention relates to a communication apparatus based on a spread spectrum communication system, and more particularly, to a communication apparatus, which carries out radio communications using a signal with a known signal for creating a delay profile added.
2. Description of the Related Art
Conventionally, the following apparatus is known as a communication apparatus, which carries out radio communications using a signal with a known signal for creating a delay profile added. Hereinafter, a case where a CDMA (Code Division Multiple Access) system is used as a spread spectrum communication system will be explained as an example.
A base station in a CDMA-based communication receives a signal on which signals of a plurality of channels are multiplexed in an identical frequency band through a transmission path at an identical time. This base station can extract a transmitted signal from each channel (each mobile station) from the reception signal by performing despreading processing using a spreading code assigned to each channel.
However, when the distance between each mobile station, which transmits a signal on each channel and the above base station, is large, a delay (hereinafter referred to as “propagation delay”) occurs by the time the signal on each channel reaches the above base station. Moreover, when the distance between each mobile station and the above base station differs from one station to another, the propagation delay also varies from one channel to another.
Therefore, the above base station needs to detect a propagation delay for every channel and perform despreading processing at timing taking account of the detected propagation delay. Therefore, conventionally, each mobile station transmits a signal with a mid amble section which is created using a known basic code added, while the base station detects a propagation delay for every channel (every mobile station) by carrying out correlation value calculation processing using the reception signal on which signals transmitted from different mobile stations are multiplexed and the above known basic code. Hereinafter, the method of detecting a propagation delay using a mid amble section in the conventional CDMA communication system will be explained.
First, the signal transmitted by each mobile station (each channel) is explained with reference to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram showing the procedure for creating a mid amble pattern in the conventional CDMA communication system. FIG. 2 is a schematic diagram showing transmission timing in each mobile station in the conventional CDMA communication system. Here, suppose there are eight mobile stations which carry out radio communications with the base station apparatus.
As shown in FIG. 1, the pattern of the mid amble section which is used for each channel (hereinafter referred to as “mid amble pattern”) is created according to the procedure shown below using a basic code which cycles for every 456 (=8 W) chips. This basic code is known to the base station and contains eight blocks A to H which has a code with mutually different W (=57) chip length.
First, as the 1st step, a reference block is set in the above basic code. Here, suppose the reference block is “A”.
As a 2nd step, the above reference block is shifted by {W×(n−1)} to the left in the figure for every channel. Here, W=57 chips and n denotes the number of the channels. The phase to be shifted is 0, W, 2 W and 7 W for channel 1, channel 2, channel 3 and channel 8, respectively. With this, the reference block on each channel is “A”, “B”, “C” and “H” for channel 1, channel 2, channel 3 and channel 8, respectively.
As a 3rd step, 513 chips are extracted from the forefront of the reference block whose phase is shifted in the 2nd step in the above basic code for every channel. This creates a 513-chip mid amble pattern for every channel as a whole. Moreover, as for each 513-chip mid amble pattern, the first one chip of the first block is removed. In this way, a 512-chip mid amble pattern is created for every channel as a whole. In FIG. 1, the first block in the 512-chip mid amble pattern created for every channel is equivalent to the last block whose first one chip is removed. For example, in the case of channel 1, first block “A′” corresponds to the last block “A” whose first one chip is removed.
Next, as shown in FIG. 2, each mobile station transmits the transmission signal with the mid amble pattern of each channel created using the above procedure added to the base station apparatus. That is, each mobile station transmits the transmission signal for which a mid amble pattern for every mobile station is added to the mid amble section between data section 1 and data section 2 at the same timing as that of the other mobile stations.
On the other hand, the base station receives a signal on which transmission signals transmitted from the mobile stations are multiplexed in a same frequency band.
Correlation value calculation processing using a reception signal in the base station and the above known basic code will be explained with reference to FIG. 3 and FIG. 4. FIG. 3 is a schematic diagram conceptually showing a situation in which the base station in the conventional CDMA communication system receives a transmission signal for every channel. FIG. 4 is a schematic diagram showing an example of a delay profile obtained by the correlation value calculation processing in the base station in the conventional CDMA communication system.
As described above, since each mobile station is distant from the base station and in addition the distance between each mobile station and the base station varies from one station to another, as shown in FIG. 3, by the time the signal transmitted by each mobile station arrives at the base station, a propagation delay is produced and moreover this propagation delay varies for every signal transmitted by each mobile station. That is, the delay times produced by the time the signal transmitted from each of mobile stations 1, mobile station 2, mobile station 3 and mobile stations 8 arrives at the base station are propagation delay 1, propagation delay 2, propagation delay 3 and propagation delay 8, respectively. The signal, which the base station receives, is a signal on which the transmission signals from the mobile stations with the propagation delays mainly shown in FIG. 3 are multiplexed.
The base station carries out correlation value calculation processing to extract a transmission signal of each mobile station from such a reception signal. Hereinafter, the correlation value calculation processing in the base station will be explained. First, of the reception signal of 512 chips received from reference time 13, only 456 chips are extracted from last part 12. Here, the reference time refers to the time that the first part (for example, first part 11 in the case of channel 1) in each mid amble section in the signal transmitted by each mobile station is received by the base station when there is no propagation delay.
Next, a value of a correlation between the extracted 456-chip reception signal and the above known cyclic basic code is calculated. That is, using the above known cyclic basic code shown in FIG. 4 as the reference, the above 456-chip reception signal is multiplied by the above basic code while shifting the phase of the above 456-chip reception signal by 1 chip at a time and a correlation value at each phase is calculated.
By such correlation value calculation processing, a delay profile on each channel as shown in FIG. 4 is obtained. During the calculation of the above correlation value, when the mid amble pattern from one of the mobile stations contained in the above 456-chip reception signal matches the above known basic code, the correlation value reaches a maximum and the path of a certain size appears.
Therefore, the time at which the size of each of path 21, path 22, path 23 and path 24 reaches a maximum corresponds to when each mid amble pattern from the mobile station 1, mobile station 2, mobile station 3 and mobile station 8 contained in the above 456-chip reception signal matches the cyclic basic code in FIG. 4.
Here, when there is no propagation delay in each mobile station, the time at which the path corresponding to each mobile station reaches a maximum is known. Therefore, the propagation delay which occurs by the time the signal actually transmitted from each mobile station reaches the base station is detected by referring to the time at which the size of the path corresponding to each mobile station when there is no propagation delay reaches a maximum. For example, the propagation delay which corresponds to each of mobile station 1, mobile station 2, mobile station 3 and mobile station 8 is detected in chip units as propagation delay 1, propagation delay 2, propagation delay 3 and propagation delay 8 as shown in FIG. 4. Propagation delay 1, propagation delay 2, propagation delay 3 and propagation delay 8 shown in FIG. 4 are propagation delay 1, propagation delay 2, propagation delay 3 and propagation delay 8 in FIG. 3 expressed on a delay profile.
Also, when the total of a propagation delay and delay dispersion in each mobile station is smaller than the W chip length, the section where a path of a certain size appears on the delay profile is decided for each mobile station. That is, in the above case, the paths, which correspond to mobile station 1 to mobile station 8, appear in the W chip sections 1 to 8 (the delay profile width) in the delay profile shown in FIG. 4.
As mentioned above, it is possible to perform interference removal and demodulation of the data section for every mobile station by carrying out despreading processing using the data section at the timing taking account of a propagation delay for every mobile station detected as shown above.
Moreover, the base station can perform time alignment control using the propagation delay for every each mobile station detected as described above. That is, the base station sets transmission timing for every mobile station based on the propagation delay for every mobile station detected and reports the transmission timing set to each mobile station, and each mobile station transmits to the base station according to the transmission timing reported by the base station. By such time alignment control, the base station can control variations of the reception timing among mobile stations.
However, as the cell radius of the above conventional CDMA communication system grows, the farther the mobile station from the base station, the greater the propagation delay of the signal transmitted from the mobile station becomes and the total of the propagation delay and the delay dispersion of this signal may become bigger than the W chip length. In this case, the path which corresponds to the above mobile station does not appear in the expected W chip section in the delay profile shown in FIG. 4, but it appears in the other W chip section. For example, in case of mobile station 1, the path, which corresponds to mobile station 1, may appear in W chip sections 2 to 8, instead of W chip section 1 shown in FIG. 4.
Moreover, in the above case, if not only the desired wave but also a delay wave of the signal transmitted from the above mobile station is received by the base station, the path of the delay wave in addition to the path of the desired wave which corresponds to the above mobile station appears in the other W chip section in the above delay profile.
As a result, because the path of the desired wave and the delay wave in the above mobile station does not appear in the expected W chip section in the delay profile obtained, the propagation delay detected in the above mobile station becomes incorrect. Also, because each path of the above mobile station appears in the W chip section which corresponds to the other mobile station in the above delay profile, there is a possibility that each path of the above mobile station will be detected mistakenly as the path of the desired wave and the delay wave of the above other mobile station. Therefore, the propagation delay detected in the mobile stations other than the above mobile station also becomes incorrect.
Therefore, because the correct propagation delay in each mobile station cannot be detected, not only the interference removal and demodulation characteristic deteriorates but also it is difficult to perform time alignment control.
To solve such a problem, there is a method of enlarging the W chip section of each mobile station in the delay profile by extending W. However, because the value obtained by dividing the mid amble section by (number of channels accommodated+1) is equivalent to the delay profile width W of each mobile station, if W is extended, the number of channels accommodated decreases supposing that the mid amble section length is constant.